Self-supporting ethylene polymer films are characterized by poor slip properties, that is, they do not readily slide over each other. This poor slip property interferes with the use of the films in automatic processing equipment because the film must be capable of freely passing through the fabricating machine, be it a heat-sealer, bag maker, bag loader or filler, bag opener or overwrap package, in order that the machine operate properly.
The ethylene polymer films are also characterized by a tendency to resist separation by a separating force applied perpendicular to the film, e.g., by a simple lifting action. This property, known as blocking, contributes to the poor handling characteristics of the film. For example, in blown tube extrusion, blocking occurs at the nip rolls used to retain the inflating gas within the tube and the temperature of the tubular film entering the nip rolls is frequently high enough to cause the film layers to stick or adhere to each other.
Ethylene polymer film layers are normally separated by a combination of sliding and lifting forces. Accordingly, it will be recognized that the slip and block characteristics of the film bear a close relationship to one another. The lifting force used during separation is resisted by the normal blocking force of the film and the sliding force is resisted by the slip forces, i.e., friction. Thus, although the blocking and sliding or slip forces are theoretically at right angles to each other, both blocking and slipping are problems encountered in the commercial processing of ethylene polymer films.
In order to improve the processability of ethylene polymer films, it is conventional practice to incorporate slip agents and anti-block agents into the ethylene polymer prior to extrusion. Over the years, numerous materials have been evaluated and polymer compositions having reasonable anti-block and slip properties have been developed. For many applications, the plastic films are required to have good optical properties, and in such formulations, a delicate balance between the base resin, anti-blocking agent and slip agent is necessary in order to obtain a high clarity film.
We have now found that improved ethylene polymer films can be obtained by replacing a portion of the anti-blocking agent conventionally employed with N,N'-ethylene-bis-erucamide. The N,N'-ethylene-bis-erucamide, hereinafter referred to as EBE, is a good anti-blocking agent but cannot be used as the sole anti-blocking agent in the resin composition because it also has a high coefficient of friction and would therefore neutralize the effect of the slip agent. We have also found that the use of EBE will improve the optical properties in high clarity film compositions.
Various amides have been proposed for incorporation into polyolefin film compositions. For example, British Pat. No. 915,589 teaches the incorporation of from 0.01-3% of an unsaturated fatty acid amide having at least 8 carbon atoms in the molecule, such as erucamide, in crystallizable linear polymers and copolymers of alpha-olefins having at least 3 carbon atoms. This British patent also teaches the additional use of 0.01-1% of a saturated fatty acid amide, for example, alkylene distearamides such as methylene and ethylene distearamides. U.S. Pat. No. 2,770,609 as well as the British Pat. No. teach that generally unsaturated fatty acid amides tend to reduce coefficient of friction (COF) whereas saturated fatty acid amides tend to reduce film blocking.
A printing and heat sealing problem has always existed when stearamide is used in polyethylene films at concentrations of greater than 0.1 weight percent. To solve this problem, inorganic materials, and particularly silicas, have been utilized instead of organic anti-block agents. The combination of an unsaturated amide such as oleamide or erucamide with an inorganic anti-blocking agent such as diatomaceous earth (see U.S. Pat. No. 2,956,035) has provided the best commercial composition to date for high speed, economical polyethylene film production. The aforementioned patent states that high clarity films having haze of less than 14% have a high tendency to block. Since this patent was issued high clarity commercial polyethylene films have haze values of less than 7% putting more emphasis on the need for effective anti-blocking agents. The use of diatomaceous silica of particle size range to be effective as an anti-blocking (see U.S. Pat. No. 3,028,355) creates a problem of reducing the clarity (haze) of the polyethylene film. This is mentioned by Mock in his August, 1974 article in Plastic Technology, pages 41-43 (see especially page 43, second column).
U.S. Pat. No. 3,021,296 teaches that ethylene bis oleamide has minimum exudation and is thereby a better slip and anti-blocking agent especially upon storage. However, in many commercial operations, the film is electronically treated for printing and sealing in-line and thus the special feature claimed is of no advantage at all. Furthermore, a printing problem is encountered with the use of ethylene bis oleamide.
U.S. Pat. No. 3,165,492 teaches that alkylene bis amides such as N,N'-ethylene bis oleamide and N,N'-ethylene bis erucamide impart desirable slip (low COF) properties to polyolefin films whereas N,N'-ethylene bis stearamide gave inferior slip properties, i.e., &gt; 0.6. U.S. Pat. No. 2,938,879 also teaches that COF values greater than 0.6 indicate poor slip, COF values of 0.6 to 0.46 indicate fair (marginal) slip and COF values of 0.45 or less indicate good slip. From Table I of U.S. Pat. No. 3,165,492 one would expect that N,N'-ethylene bis erucamide used along would impart good slip properties. However, this was not found in our work; it was found that a good slip agent such as erucamide was necessary to impart "good" slip properties.
The use of EBE is distinct from such prior art as will become apparent in the following description. For example, in view of the necessity of using silica as the anti-blocking agent to avoid the problem in printing on the film and in heat sealing, the discovery that EBE could replace a portion of the silica thereby improving optical properties (haze) while maintaining satisfactory resistance to blocking and without detrimentally affecting the printability of the polyethylene film was indeed surprising.
It is the object of this invention to provide a new composition which will produce ethylene polymer films having improved properties and particularly ethylene polymer films of high optical clarity. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description.